Many current printed circuit board constructions require one or more external conductive layers, e.g., circuitry and/or pads for mounting components thereon, and, given today's increased functional demands, a plurality of internal conductive planes, e.g., signal, power and/or ground. To provide effective interconnections between components and the board's conductive circuitry and pads, the use of through holes has been adopted wherein several such holes are passed through the board and electrically coupled in a selective manner to internal and external conductive elements. Such holes typically include a conductive, e.g., copper, layer as part thereof which in turn contacts the also typically copper circuitry and pads.
The term "through hole" or simply "hole" as used herein is meant to include both conductive and non-conductive apertures which may extend entirely through the circuit board or only partly therethrough (such holes are often called "vias" in the art), including between only one or more internal layers without being externally exposed. Examples of various circuit board structures which include the use of holes of these types and various methods of making such holes in circuit boards are defined in several published documents, including the following U.S. Letters Patents, issued on the dates identified:
Printed circuit boards of the aforementioned type are particularly adapted for having one or more (usually several) electrical components, e.g., semiconductor chips, capacitors, resistors, etc., mounted on an external surface thereof and coupled to various, selected internal conductive planes within the board's dielectric substrate. As demands for increased levels of integration in semiconductor chips and other electrical components continue, parallel demands call for concurrent increased functional capabilities, e.g., increased circuit densities, in printed circuit boards adapted for use with such components. Such demands further emphasize the growing need for more closely spaced electrical components on the board's outer surfaces. For those boards possessing greater functional capabilities and therefore which use several through holes therein, it is highly desirable to position the electrical components directly over the holes to maximize board real estate while assuring a compact, miniaturized final board product.
Demands such as those above are particularly noteworthy when it is desirable to couple what are referred to as ball grid array (BGA) components directly onto the board's outer conductive layer(s). Such components include a plurality of highly dense conductors, e.g., solder ball elements, closely positioned in a fixed pattern on the component's undersurface. Such is also the case for directly mounted semiconductor chips (also known as direct chip attach or DCA components) wherein a dense pattern of several minute solder balls are arranged on the chip's compact and extremely small undersurface (that directly facing the underlying circuit board). To successfully accommodate such components, filling of the board's holes with conductive material (called "fill") has been tried, with one or more examples defined in the above-identified issued Letters Patents. To accomplish such filling, however, it is typically necessary to utilize a mask or the like with pre-formed, e.g., drilled or punched, apertures therein which coincide with the desired hole pattern. Once provided in the necessary precise alignment, various filler materials have been attempted, including both electrically conductive and non-conductive. One example of such a filler is defined in U.S. Pat. No. 5,487,218, wherein the composition is an organic polymeric material with an optional particular filler added thereto to modify the thermal and/or electrical properties of the composition. One excellent reason for such modification is to approximately match the coefficients of thermal expansion of both board substrate material and contained filler.
In an approach somewhat different from using an apertured mask, a rupturable supporting member having fill thereon is used. Pressure is applied to the fill, resulting in rupturing of the support layer in the vicinities of the circuit board substrate's holes. Fill thus fills these holes and the support member and fill remainder are removed. This approach is defined in copending application Ser. No. 09/076,649, mentioned above.
It is believed that a new and unique method of making a printed circuit board having at least one (and possibly several) holes therein which can be filled in a more expeditious manner than heretofore described wherein performed masks or similar structures are used would represent a significant advancement in the art. It is further believed that a printed circuit board produced in accordance with such a method would also constitute a significant art advancement, as would a fill member usable during the method for providing expeditious hole fill.